Magnetic recording media, e.g., floppy disks, rigid disks and tape, record information in "tracks". The number of such tracks per inch is in large part a function of the ability to reliably and reproducibly locate particular tracks and ensure the read/write head accurately follows a track once it has been located. Conventional 51/4" floppy disks usually have 48 or 96 tracks per inch ["tpi"], while 31/2" floppy disks have 135 tpi. In contrast, a Winchester disk typically carries 1,000 tpi, but special means must be provided to reliably servo, i.e., control movement of the read/write head and hold it on track. It is very desirable to increase the number of tracks per inch for floppy magnetic media, so that appreciably more information may be stored on the same area.
As the number of tracks per inch increase, the space between tracks decreases, thus requiring a narrower read/write head width as well as more precise tracking to avoid reading/writing on more than the intended track. The read/write head should be centered on the track and not read or write on adjacent tracks. If the magnetic recording medium is a floppy disk, the problem of accurate tracking is made harder by anisotropic dimensional changes, e.g., in the typical polyester support, as a function of temperature and humidity changes, possibly resulting in an "elliptical" track configuration in place of the desired circular track; by vibration of the disk during high speed rotation changing the relative position of the disk to the head or preventing the disk from being truly flat; off-center positioning due to wear of the disk center hole with repeated use; off-center positioning when used in different drives, etc. Indeed, the "on center" positioning of the center hole during manufacture of the disk itself is subject to variation.
The art has recognized the need for higher tpi values, and a number of approaches to the requisite tracking ability have been tried with varying degress of success.
One approach to this problem is disclosed in U.S. Pat. No. 3,130,110 issued Apr. 21, 1964 to Schmidt, wherein it is proposed to cut spiral grooves into part of the disk to provide tracking guidance. Embossed (raised) spiral grooves to provide a guide pattern are disclosed in U.S. Pat. No. 3,772,081 to Franer.
Japanese Kokai (Published Patent Application) No. 59-146444 published Aug. 22, 1984, proposes to leave outer and inner areas of a magnetic disk uncoated with magnetic material, and to use a light transmission type optical sensor to detect the magnetic area edge and effect alignment to facilitate track position following.
U.S. Pat. Nos. 4,516,177 issued May 7, 1985 to Moon, et al.,4,396,959 issued Aug. 2, 1983 to Harrison, et al., and 4,419,701, issued Dec. 6, 1983 to Harrison, et al. (all assigned to Quantum Corporation), propose to use an optical encoder to provide "coarse" servo control of the read/write head to position the head close to a track, the disk having factory pre-recorded thereon radial sector bursts to provide centerline correction information to provide fine correction and keep the read/write head in centerline alignment with the desired track. The optical encoder (see, e.g., FIG. 5 of Harrison, et al., U.S. Pat. No. 4,396,959) comprises a light source, a scale having a series of equally closely spaced microscopic radial lines, and an integrated circuit photosensitive reticle-masked array, which in combination produce light and dark polyphase (quadrature) patterns used to generate a servo waveform. The optical encoder is completely separate from the magnetic disk.
IBM Technical Disclosure Bulletin, Vo. 27, No. 8, January 1985, pp. 4877-4878, entitled "Servowriter Reference Clocks from Shaft Encoder", discloses a moire fringe optical encoder mounted directly to the main drive spindle close to, but separate from, the magnetic disk.
U.S. Pat. No. 3,633,038 issued Jan. 4, 1972 to Falk discloses an optical system for positioning a transducer in registration with a track on a magnetic tape or disk. A pair of optical masks having alternating opaque and transmissive lines are positioned so that light passing through both masks strikes a pair of photocells. One optical mask is mounted on the transducer carriage so as to move across the second optical mask as the transducer is moved laterally across the magnetic tracks. The light striking the photocells generates a signal which may be used to identify the track position of the transducer. Again, the optical masks are separate from the magnetic media.
U.S. Pat. Nos. 4,558,383 issued Dec. 10, 1985 to Johnson and 4,587,579 issued May 6, 1986 to Cocke, et al. disclose magnetic media bearing markings which can be detected optically by reflection to provide a servo signal. U.S. Pat. No. 4,570,191 issued Feb. 11, 1986 to DiStefano, et al. discloses optical sensors suitable for mounting in the slider housing the read/write head, the optical sensor being used to detect such reflected optical servo tracks or indica positioned on the magnetic media, as contemplated, inter alia, by said Johnson and Cocke, et al. patents.
IBM Technical Disclosure Bulletin, Vol. 16, No. 9, February 1974, p. 3020, discloses an "Optical Servo Technique Using Moire Fringes", wherein a grating of opaque or non-reflecting concentric circles with equal reflecting spaces is formed on the magnetic storage hard disk (reducing the area available for magnetic recording), and a grating of parallel opaque or non-reflecting lines of the same dimensions is provided on a transparent member positioned on the slider carrying the read/write head. Light from a light-emitting diode [LED], also positioned on the slider (or arm) carrying the read/write head, is reflected off the disk grating to form a moire pattern with the slider grating if the slider is skewed by a few degrees with respect to the disk. A sinusoidal optical density variation produced by changes in the moire pattern can be detected by photodiodes, and phase changes in the sinusoidal output can be interpolated by suitable electronic circuitry to provide directional information to position the head over the desired track.
Other approaches to solving the problem include providing magnetically recorded, spaced servo track information, e.g., the so-called "embedded servo". The servo signals typically divide the disk into sectors, with the result that recording is not on a continuous track and servo information is not continuous. In addition, the magnetic area available for recording information is reduced by the area dedicated to the magnetic servo information. Yet another approach is disclosed in PCT Published Application WO No. 85/02933 published July 4, 1985, wherein it is proposed to provide servo information by the use of optical recording tracks positioned between magnetic tracks to provide optical guidelines readable by reflected light.
Most, if not all; of the prior systems noted above suffer from the disadvantage of reducing the magnetic surface area available for magnetic recording. In addition, the information signal used for servo purposes is usually discontinuous, resulting in intermittent servo changes.